Normal map curved edges with black shading
This is caused by the fact that the normal color (let's say R: 127, G: 192, B: 255 -> aka a normal pointing up at about 45° when you view the bitmap normal map in image editor) which defines the particular little space on the actual polygon which is also pointing upwards, let's say at a 88° angle compared to the camera view vector. The resulting angle of the vector pointing up for that little tiny space on the polygon is then 88° based on geometry +45° based on normal map which is far more than 90° and thus looks black since the ray as far as Cycles is concerned basically goes into the polygon.



Normal Map used in the above picture:


One way to "fix" this is by using fresnel to gradually reduce normal map strength at grazing angles. This won't help you in Game Engines I suppose but as far as I've seen they handle this little problem well as I only encounter it in rendering engines.